This proposal is concerned with: (1) bone remodeling and associated structural changes in the proximal femur subsequent to implantation of porous coated femoral components in cementless total hip arthroplasty (THA) and (2) revision of failed cemented total hip arthroplasty with porous coated femoral components. The premise of bone ingrowth fixation is that the prosthesis will remain firmly anchored to the host bone indefinitely. Consequently, complications due to bone remodeling contitute one of the major long-term concerns. The location of the porous coating and the stiffness of the stem are two of the design parameters likely to influence the mechanical environment and, hence, remodeling of the proximal femur subsequent to cementless porous coated THA. A canine model will be used to provide data on the effect of these two parameters on bone remodeling and structural changes following cementless THA. Radiographic evidence of aseptic loosening has been reported to be as high as 40% in cemented revision of failed cemented THA. Cementless porous coated prosthesis are being used for revision arthroplasty, but it is not known if bone ingrowth actually occurs in this situation nor are the long-term effects on bone remodeing known. A canine revision model and a canine model of a gap at the interface between the host bone and the porous coating will be used to provide insight into the process of bone ingrowth, into the possibility of enhancing this process, and into cortical bone remodeling following cementless revision THA.